1. Field
The present disclosure relates to amplifiers. More specifically, it relates to a stacked pre-driver amplifier and to devices using a stacked pre-driver amplifier, such as power amplifiers having a stacked pre-driver amplifier.
2. Description of Related Art
Radio frequency power amplifiers are commonly used in many applications. Often, they are used in consumer battery-powered applications such as mobile telephones, for which power efficiency is a very important attribute.
As semiconductor processes shrink to smaller dimensions in order to achieve increased performance, reliability criteria cause a reduction in the maximum allowed voltage across the terminals of a transistor. This maximum allowed voltage is defined as Vuse. Common battery voltages can exceed 3.5V, while maximum transistor voltages may be 2.5V or even less. The voltage imposed on a pre-driver stage of a power amplifier must be reduced from the battery voltage down to Vuse. This may be accomplished by several means known to one skilled in the circuit design arts. 1) DC-DC voltage converters can be used to provide a lower voltage, but power is lost in the conversion efficiency and there is significant cost in creating a DC-DC converter. 2) A resistor divider can be used to reduce the voltage from the battery voltage to the use voltage, but the power dissipated in the resistor divider is wasted as heat, resulting in reduced efficiency of the overall amplifier circuit. 3) Similarly, a transistor having a variable bias can be used to provide a voltage drop that can be changed as desired. However, the power dissipated in the transistor is also wasted as heat, resulting in reduced efficiency.
The above problem is illustrated in FIG. 1, which shows a block diagram of a prior art radio frequency power amplifier (PA) with a pre-driver circuit (102). The pre-driver circuit (102) comprises a pre-driver amplifier (104) and transistors (106), (108). Regulator (110) is in series with transistors (106) and (108) between a power supply voltage Vbat and a reference voltage Vref. The function of regulator (110) is familiar to one skilled in the circuit design arts and could be any of several different designs. For example, it could be a stack of p-channel transistors, or any of several other circuit topologies.
Vbat can be, for example, 3.5V (as is common with mobile telephones). On the other hand, as also described above, a typical 0.25 μm CMOS process has a maximum voltage (Vuse) allowed across any two terminals of a transistor of 2.5V. Such limitation is due to physical degradation effects known to one skilled in the art of semiconductor device design and may include gate oxide breakdown due to so-called time-dependent dielectric breakdown (TDDB) or so-called hot carrier injection (HCI). These physical phenomena cause degradation of device performance of the system or even failure of the system. The rule for the maximum voltage between any two terminals of a transistor should therefore be strictly followed in order to meet reliability requirements of the system.
Referring again to the circuit in FIG. 1, if Vbat is 3.5V and Vref is electrical ground, the power supply differential voltage is 3.5 Volts. Vin (112) is an external source signal that increases and decreases in value over time and is capacitively coupled to the driver amplifier (104). The driver amplifier (104) outputs a signal that is high when Vin (112) is high and low when Vin (112) is low. The highest voltage that can be output from the driver amplifier (104) is Vpre. The lowest voltage that can be output from the driver amplifier (104) is Vref.
As is typical with CMOS inverter arrangements, when the output of the driver amplifier (104) is close to Vpre, the p-channel transistor (106) is turned OFF and the n-channel transistor (108) is turned ON. As a consequence of this, the input voltage Vg1 to the PA (114) is pulled down close to Vref.
At this point, if it is assumed that Vdrop of regulator (110) is close to zero, then the voltage present at the source of the p-channel transistor (106) is approximately Vbat and the voltage present at the drain of the same p-channel transistor (106) is approximately Vref. Therefore, if Vbat is assumed to be equal to 3.5V and Vref is assumed to be equal to electrical ground, then Vds=Vdrain−Vsource of the p-channel transistor (106) is approximately 3.5V. If the above mentioned rule for Vuse sets a maximum amount of 2.5V, then this situation violates the rule for Vuse.
On the other hand, when the output of the driver amplifier (104) is close to Vref, the p-channel transistor (106) is turned ON and the n-channel transistor (108) is turned OFF. As a consequence, the input voltage Vg1 to the PA is pulled up close to Vpre.
If Vdrop of the regulator (110) is assumed to be close to zero, then Vds of the n-channel transistor (108) is close to Vbat−Vref. Therefore, if Vbat is 3.5V and Vref is 0V, then Vds of the n-channel transistor (108) is close to 3.5V, which violates the Vuse rule.
As noted above, one possible solution is to increase Vdrop, for example to 1.3 V, by changing the Vbias of the regulator (110). Should this happen, the Vds of the p-channel transistor (106) or the n-channel transistor (108) is reduced to 2.2V, thus satisfying the Vuse rule. However, such a possible solution increases power dissipation in the regulator (110). Such power would be wasted as heat, reducing the overall efficiency of the power amplifier (PA).